1. Field of the Invention
The present invention relates to an image display apparatus which uses a display panel, such as a plasma display panel, that displays an image in a multi-level gray scale by dividing one TV field of the image into a plurality of subfields, and especially to an image display apparatus for reducing halftone disturbance which occurs when displaying a moving image.
2. Description of the Prior Art
Image display apparatuses which use display panels that have two display states in which each pixel can be "on" or "off", represented in this specification by plasma display panels (hereinafter simply referred to as "PDPs"), produce gray-level images by display methods such as the Address Display Period Separated Sub Field method. In this method, an image is displayed by dividing the time in one TV field into a plurality of subfields that are each composed of an addressing period in which "on"/"off" data is written for each line of a PDP screen and a discharge sustaining period in which predetermined pixels are illuminated all at once.
It is conventionally known that when displaying a moving image in a multi-level gray scale by dividing each TV field of the moving image into a plurality of subfields, gray level disturbance in the form of false-edge appears on the screen.
The following is an explanation of an occurrence of such false-edges when displaying a moving image, with reference to FIGS. 18 and 19. FIG. 18 shows movement of a picture pattern PA1 on a screen of a PDP 180, the picture pattern PA1 being composed of two pairs of adjacent pixels having the similar gray levels 127 and 128 respectively. In this example, the picture pattern PA1 moves by two pixels per TV field. In FIG. 19, the horizontal axis shows a relative position of each pixel on the screen, and the vertical axis shows a period which for convenience's sake corresponds to one TV field. FIG. 19 also shows how the movement of the picture pattern PA1 appears to a viewer. Here, a case is explained in which each 8-bit gray level, that is, each of 256 gray levels, is converted into 8-bit data showing "on"/"off" states of eight subfields, which is then used for displaying the corresponding gray level. As a specific example, the time in one TV field is divided into subfields 1-8 which are assigned weightings of 1, 2, 4, 8, 16, 32, 64 and 128, respectively (in ascending order). In this case, the gray level 127 can be displayed by lighting the subfields 1-7 (diagonally shaded areas on the left in FIG. 19) and not lighting the subfield 8, while the gray level 128 can be displayed by not lighting the subfields 1-7 and lighting the subfield 8 (diagonally shaded area on the right in FIG. 19). Though in FIG. 19 the lighting of each subfield appears to be continuous for a predetermined length, each subfield in the PDP is actually composed of a set of pulse illuminations corresponding to its weighting value, so that each subfield has an interval that is equal to the addressing period.
When displaying a static image, the average luminance of one TV field of the observed image is expressed by the integral of the lighting periods between A-A' in FIG. 19, so that the desired gray level is properly displayed. On the other hand, when displaying a moving image, an integral of the lighting periods of either B B' or C-C', depending on the direction followed by the eye, is observed on the retina. The total value of each bit (subfield) between B B' is approximately 0, while the total value of each bit (subfield) between C-C' is approximately 255. Thus, when observing the movement of a picture pattern in which two similar gray levels, such as the gray levels 127 and 128, are adjacent, the gray levels in the level changing part appear profoundly disturbed due to the movement of the image as shown in FIG. 19.
As explained above, a halftone is represented by an integral of luminance values of each subfield in a time series. Accordingly, when the eye follows a moving image, weighting values of bits which are in a different position from the original pixel position are integrated, and as a result the halftone display appears profoundly disturbed. It should be noted here that this halftone disturbance appears as false-edges in the image, and so generally referred to as the "moving image false-edge". Such false-edge occurrences in a moving image display are explained in detail in Hiraki Uchiike and Shigeru Mikoshiba All About Plasma Display Kogyo Chosakai, Tokyo (May 1, 1997): pp. 165-177.
In order to eliminate moving image false-edges and reduce halftone disturbance in a moving image display, an attempt has been made with conventional image display apparatuses to divide a total weighting value of the subfields 7 and 8 which correspond to the high-order bits and intersperse the divided parts in the first and second halves of one field. FIG. 20 shows a subfield construction in a conventional method for reducing the moving image false-edges by using ten subfields to display 8-bit gray levels, that is, 256 gray levels. These subfields are assigned weightings of 48, 48, 1, 2, 4, 8, 16, 32, 48, and 48 in order of time. That is to say, the combined weighting value of 64 and 128 for the high-order subfields 7 and 8 out of the eight subfields described above is divided into four equal weightings ((64+128)/4=192/4=48.times.4), which are then interspersed in the first and second halves of one field to prevent the occurrence of the halftone disturbance by reducing the weighting values of the high-order subfields. With this technique, halftone disturbance is scarcely observed in the gray level changing area between 127 and 128 described above, so that the occurrence of the moving image false-edges can be prevented for those values. However, for a different example, like the gray level change from 63 to 64 shown in FIG. 20 in which a subfield with a large weighting (here, the subfield 9) is turned "on" for the first time while subfields with small weightings (here, the subfields 3, 4, 5, 6, and 8) are turned "off", the distribution of "on" subfields and "off" subfields is greatly changed. As a result, halftone disturbance is inevitably observed in the level changing area. As shown in FIG. 20, a gray level observed in the direction of the arrow (a) is approximately 79, while a gray level observed in the direction of the arrow (b) is approximately 32. Thus, it is still not possible to prevent the occurrence of the moving image false-edges when displaying a moving image in such gray levels.
Also, when using such a subfield dividing method that divides the total weighting value of the high-order bits and intersperses the divided parts in the first and second halves of one TV field, four subfields with a weighting of "48" which have been interspersed in a time series within the TV field comprise a weighting value of 192, occupying a large portion of the total weighting value of 255. Usually, an observed image is composed of image components which are interspersed in the time series. Thus, in the subfield dividing method which intersperses each subfield with a large weighting in the first and second halves of one field, when displaying a moving image in a high-luminance area using these subfields, the observed image will be a composition of image components which are displayed using the subfields with the weighting of "48". This often causes the moving image to appear blurred. The appearance of such blurs in the moving image display affects picture quality, such as by making small moving characters appear double so that they cannot be read.